US3313762A - Organopolysiloxane elastomers containing peroxy carbonate curing agents - Google Patents

Description

United States Patent 3,313,762 ORGAN (JPOLYSILOXANE ELASTOMERS CON- TAINING PEROXY CARBONATE CURING AGENTS Charles W. Pfeiler, Schenectady, N.Y., assignor to General Electric Company, a corporation of N ew York No Drawing. Filed Aug. 29, 1963, Ser. No. 305,534 5 Claims. (Cl. 260-37) The present invention relates to organopolysiloxane compositions convertible to the cured, solid, elastic state and to the elastomers produced therefrom. More particularly, the present invention relates to the employment of a tertiary-alkylperoxyalkylcarbonate as a curing agent for organopolysiloxane compositions convertible to the cured, solid, elastic state.

Molded organopolysiloxane elastomers have been employed in a variety of applications requiring materials with a high degree of toughness, and an ability to resist permanent deformation resulting from compression at ele- Vated temperatures over an extended period of time. One of the principal factors determining the performance of such organopolysiloxane products, which can be molded in the shape of gaskets, oven seals, etc., is the curing agent used during the fabrication stage. Generally, in applications in which toughness of the molded organopolysiloxane product is critical, benzoyl peroxide is often preferred. Dicumyl peroxide is utilized more often as a curing agent for making organopolysiloxane elastomers possessing toughness, as well as dimensional stability under periodic or continuous stress at elevated temperatures. However, those skilled in the art know that substantially higher curing temperatures are required to obtain effective results with dicumyl peroxide as compared to benzoyl peroxide. Because of this higher temperature requirement, fabricators oftenfind that molding with dicumyl peroxide is economically less attractive, since molding is usually accomplished by use of high pressure steam. Again, even though both benzoyl peroxide and dicumyl peroxide provide for the production of molded organopolysiloxane elastomeric products having a high degree of toughness, it has been found that the toughness of these products falls off significantly after being heated to an elevated temperature after 24 hours or less.

It would be desirable therefore to be able to make a tough organopolysiloxane elastomer having a satisfactory ability to resist permanent deformation after being compressed at elevated temperatures by molding an organopolysiloxane composition at moderate temperatures, for example, temperatures at about 150 C. or below. In addition, it would also be desirable to produce by such method, an organopolysiloxane elastomer having improved resistance to heat-age.

The term toughness as utilized hereinafter with respect to describing the elastomers of the present invention is defined as the product of the tensile strength (p.s.i.), and elongation (percent) of the elastomer. The term heatage is defined as the change in physical properties of the elastomer, such as a loss in tensile or elongation resulting from heating the elastomer to an elevated temperature for an extended period of time. Compression Set is the permanent decrease in thickness of a rubber sample which results after the sample has been compressed to a certain predetermined thickness at an elevated temperature over an extended period of time. It is expressed as a percentage of the original deflection.

The present invention is based on the discovery that certain tertiary-alkylperoxyalkylcarbonates are valuable curing agents for molding organopolysilixane compositions to the elastomeric state at temperatures significantly depending upon the method of manufacture.

below that required for dicumyl peroxide. In addition, the resulting molded products exhibit superior toughness, ability to resist change in physical properties due to heatage, and have a low degree of compression set.

The tertiary-alkylperoxyalkylcarbonates that have been found operable in the present invention have the formula,

(R)sCOO( JOR (2) 10 to 300 parts of filler, and (3) 0.001 to 10 parts of a peroxide shown by Formula 1, where R is a member selected from monovalent hydrocarbon radicals, halogenated monovalent hydrocarbon radicals, and cyanoalkyl radicals, and a is equal to 1.95 to 2.01, inclusive.

Radicals included by R of Formula 2 are for example, aryl radicals and halogenated aryl radicals, such as phenyl, chlorophenyl, xylyl, tolyl, etc.; aralkyl radicals such as phenylethyl, benzyl, etc.; aliphatic, haloaliphatic, and cycloaliphatic radicals such as alkyl, alkenyl, cycloalkyl, haloalkyl, including methyl, ethyl, vinyl, ally, propyl, chlorobutyl, cyclohexyl, trifiuoropropyl, etc.; cyanoalkyl radicals such as cyanoethyl, cyanopropyl, cyanobutyl, etc.

The fillers that are employed in the organopolysiloxane compositions convertible to the cured, solid, elastic state are known to the art as reinforcing and semireinforcing fillers. The reinforcing fillers, such as the silica fillers, including fumed silica, precipitated silica and the like, are structure inducing and depending upon their manufacture, can contain or be free of hydroxyl groups either in the form of adsorbed moisture, or bonded to silicon atoms. These structure inducing fillers can be modified such as, for example, by the introduction of silicon-bonded alkoxy groups or silicon-bonded alkoxy radicals in place of some hydroxy radicals, resulting in certain advantages, such as decreased structure, when incorporated into the organopolysiloxane polymer.

The preferred silica filler of the present invention is fumed silica filler made by fuming processes including the vapor phase burning of silicon tetrachloride or ethylsi-licate, an example being what is known to the trade as Cab-O-Sil. Examples of other silica reinforcing fillers may be found described in US. Patents 2,541,137, 2,- 610,167, and 2,657,149. Such fillers may be slightly acidic or alkaline (that is, have pHs below or above 7) Examples of semireinforcing or usually non-structure forming types, are titanium oxide, lithopone, calcium carbon-ate, iron oxide, and diatornaceous earth.

The organopolysiloxane polymers of Formula 2 are well known in the art, and are shown, for example, in Algens Patent 2,448,756, Sprung et al. Patent 2,448,556, Sprung Patent 2,484,595, Krieble et al. Patent 2,457,688, Marsden Patent 2,521,528, Hyde Patent 2,490,357, and

Warrick Patent 2,541,137. It will, of course, be understood by those skilled in the art that these organopolysiloxane polymers can contain the same or different silicon-bonded organic substituents (e.g., methyl, ethyl, propyl, vinyl, allyl, phenyl, tolyl, xylyl, Benzyl, phenylethyl, naphthyl, chlorophenyl, cyanoethyl, both methyl and phenyl, etc. radicals) connected to the silicon atoms by carbonsilicon linkages.

The polymers can be viscous masses or gummy solids, depending upon the state of condensation of the starting organopolysiloxanes, polymerizing agents, etc., and can be prepared by condensation of a liquid organopolysiloxane containing an average of about 1.95 to 2.01, preferably, from about 1.98 to 2.01 organic groups per silicon atom. The polymerizing agents that can be employed are well known in the art and include for instance, ferric chloride hexahydrate, phenyl phosphoryl chloride; alkaline condensing agents such as potassium hydroxide, sodium hydroxide, etc.

The starting organopolysiloxanes used to make the organopolysiloxane polymers of Formula 2 consists essentially of units of the structural formula R' SiO, where R is as previously defined. For example, the organopolysiloxanes of Formula 2 can consist essentially of chemically combined (CH SiO units, or a copolymer of dimethylsiloxane with a minor amount (e.g., from 1 to 2.0 or more mole percent) of chemically combined units such as (C H (CH SiO), (C H SiO or (NCCH CH (CH SiO units or mixtures thereof.

Where alkenyl radicals are attached to silicon by carbon-silicon linkages in the polymers of Formula 2, it is preferable that the alkenyl radicals (for instance, vinyl groups, allyl groups, etc.) be present in an amount equal to from 0.05 to 2 mole percent of the total number of organic radicals in the organopolysiloxane polymer that are attached to silicon through carbon-silicon linkages.

Various other ingredients such as structure additives, pigments, heat stabilizers, etc. for example, that can be utilized are heat stabilizers, such as iron oxide, or aryl urethanes in amounts of up to 4 parts of heat stabilizer per 100 parts of polymer. Structure additives such as silanol-stopped polydiorganosiloxanes for example, polydimethylsiloxanes, alkoxy-stopped polydiorganosiloxanes, diphenylsilanediol, etc., also can be utilized.

The organopolysiloxane compositions of the invention can be made by blending together, such as by milling, doughmixing, etc., the polymer, filler, tertiary-a-lkylperoxyalkylcarbonate and other ingredients. Preferably the tertiary-alkylperoxyalkylcarbonate is utilized at from .01 to 5 parts per 100 parts of polymer. The order of addition of the various components is not critical. It is preferred, however, to add the tertiary-alkylperoxyalkylcarbonate to the polymer along with or after the filler has been added. If desired, other curing catalysts such as benzoyl peroxide, tertiary-butylperbenzoate, bis-(2,4-dichlorobenzoyl)peroxide, dicumyl peroxide, can be employed along with the tertiary-alky-lperoxyalkylcarbonate to achieve special curing efifects. Thereafter the organopolysiloxane composition can be molded at pressures from about 100 to 2,000 p.s.i. or more, in combination with temperatures ranging from about 100 C. to 300 C. or higher. Under such conditions, the time required for effecting the desired cure will depend upon such factors as the amount of tertiary-a1kylperoxyalkylcarbonate utilized. The nature of the organopolysiloxane polymer, the type and amount of filler, the use desired, etc. Persons skilled in the art will have little difficulty in determining optimum amounts of the materials utilized for particular applications.

In order that those skilled in the art can better understand the practice of the present invention, the following examples are given by way of illustration and not by way of limitation. All parts are by weight.

Example I An crg-anopolysiloxane composition was made by milling together 71 parts of an organopolysiloxa e p y having a viscosity of about 50 million centipoises at 25C., about 2 parts of iron oxide, 23.5 parts of fumed silica, 3.5 parts of a silanol-stoppedmethylphenylsiloxane having a viscosity of about 15 centipoises at 25 C., and 0.2 part of t-butylperoxyisopropylcarbonate. The organopolysiloxane polymer was chain-stopped with dimethylvinylsiloxy units, and it was composed of a major proportion of dimethylsiloxy units chemically combined with a minor proporation of diphenylsiloxy units and methylvlnylsiloxy units.

Other organopolysiloxane compositions were prepared following the same procedure containing from about 0.01 part to about 1 part of t-butylperoxyisopropylcarbonate per parts of organopolysiloxane composition. The various compositions were milled into sheets, and slabs were cut from the respective sheets.

In addition to the above organopolysiloxane compositions, other compositions were prepared in accordanc with the same procedure. In place of the t-butylperoxyisopropylcarbonate, there was utilized about 0.8 part of a mixture of equal parts of benzoyl peroxide and a poly dimethylsiloxane gum, per 100 parts of the organopolysiloxane composition. Another composition was made in which there was utilized about 0.9 part of a mixture containing dicumyl peroxide and an inert carrier, in which dicumyl peroxide was present at 40% by weight, per 100 parts of the organopolysiloxane composition. Slabs were also formed from these organopolysiloxane compositions following the same procedure.

The various slabs were then molded for 10 minutes at C. The physical properties of the molded slabs were then measured. These molded slabs were then heat-aged for 24 hours at 250 C. The slabs were then measured for compression set after the cured slabs had been heated for 70 hours at about 149 (3., while compressed to 75% of their original thickness.

In Table 1 below, the physical properties of the various slabs are shown after press-cure and heat-age. In the table, II is Hardness (Shore A), T is Tensile (p.s.i.), E is Elongation (percent), and CS. is Compression Set (percent). Under Peroxides in the table, BPIC is tertiary-butylperoxyisopropylcarbonate, Benzoyl is benzoyl peroxide, and Dicumyl is dicumyl peroxide. The parts of peroxide as shown are expressed at 100% concentration.

TABLE I Part/100 Press-Cure Heat-Age Peroxide Parts of 0.8.

Oomposition H I T E H T E BPIO 0. 1 40 1, 525 760 52 1, 500 590 Benzoyl 04 45 1, 683 750 53 1, 330 520 Dlcumyl 036 50 1, 470 510 58 1, 280 380 14 Example 2 In accordance with the procedure of Example 1, 100 parts of organopolysiloxane composition were made utilizing a trimethylsiloxy chain-stopped polymer having a viscosity of about 50 million centipoises at 25 C., and composed of a major proportion of dimethylsiloxy units chemically combined with a minor proportion of methylvinylsiloxy units. In addition to polymer, fumed silica, and iron oxide, there were utilizer, per 100 parts of the composition, 0.2 part of tertiary-butylperoxyisopropylcarbonate, and 3.5 parts of a silanol-stopped dimethyl oil 6 having a viscosity of about centipoises at 2 C. The TABLE In organopolysiloxane composition was milled mto sheets from which slabs were cut. These slabs were molded, Ptesscme Heat Age heat-aged and tested for compression set following the I Peroxide Parts procedure of Example 1. 5 H T E H T E Example 3 BPIC 0.2 53 1, 340 660 67 1,610 460 0. 45 56 1, 330 520 66 1, 330 340 An organopolysiloxane composition was made by Dleumyl 25 169 1,109 57 640 milling together a mixture of 71 parts of a polydimethyl- 10 siloxane chain-stopped with trimethylsiloxy units having a viscosity of about 30 million centipoises at 25 C., 24 Based on the results shown in the tables above, those parts of fumed silica, about 2 parts of iron oxide, and Skilled in the art would know that tertiary-alkyl-peroxy- 3 parts of tertiary-butylperoxyisopropylcarbonate. Test alkylcarbonatesof the present invention provide for the slabs were made and tested in accordance with the proceproduction of organopolysiloxane compositions condure of Example 1. vertible to organopolysiloxane elastomers possessing su- In addition to the compositions of Examples 2, and 3, periortflughness and1 ablility to resist change due heabtsimilar organopolysiloxane compositions were prepared age. e organopo ysi oxane compositions provi ed y following the same procedure respectively except that 0.8 the present invention also allow for the production of part of dicumyl peroxide, as utilized in Example 1, was elastorners at unusually low curing temperatures; also, substituted for the tertiary-butylisopropylcarbonate. the elastomers produced thereby have superior resistance Table II below shows the data obtained after the various to permanent set due to compression at elevated temperaslabs were tested following the procedure of Example 1. tures. For example in Table I, elastomers made with In Table II the terms employed are the same as shown tertiary-butylperoxyisopropylcarbonate show an outin Table I. In addition, the composition of Example 2 standing ability to resist change due to heat-age. In addiis shown as methylvinyl, and the composition of Extion, those skilled in the art know that a tensile of over ample 3 is shown as dimethyl. 1600, as shown for example for the organopolysiloxane TABLE II Press-Cure Heat-Age Peroxide Parts Composition (LS.

(100 Parts) H T E H T E BPIC 0.2 Methylvinyl 56 1, 250 580 68 1,115 320 21 3 Dimethyl 38 870 550 57 870 220 45 Dicumyl 0. 32 Methylvinyl 53 1,270 650 68 765 250 24 In addition to the results shown in Tables I and II composition containing 0.2 part of tertiary-butylisoproabove, it \vas fcpund that [536 organolpollysiloxane comfpoii- 40 pylcarbonate, ifs I:quite llIlllSllllal for organolpolysiloxaane tions pro uce in accor ance wit t e practice 0 t e compositions o t e nature s own in .EXampe 1. Ta e invention as shown in Example 1, containing as little as I also shows the unusually low compression set resulting 0.01 part and 0.02 part of tertiary-butylperoxyisopropylfrom the use of the tertiary-butylperoxyisopropylcarcarbonate per 100 parts of the organopolysiloxane combonate as compared to benzoyl and dicumyl peroxide. It position, cured satisfactorily. The cured elastorners acalso is evident from the results shown in both Table I tually sh imPIOVemeBt in toughness after e h and Table II, that the organopolysiloxane compositions age treatment instead of loss in toughness. of the present invention generally resist change due to In order to further demonstrate the advantages achieved heat-age to a much greater degree than those organopolyb employing tertiary-a1kylperoxyalkylcarbonates in Orsiloxane compositions containing benzoyl peroxide, or gehopolysiloXahe eol'hposlhehs eohverhhle 0 the e135- dicumyl peroxide. Particularly significant also are the State in accordance Wlth the p ff the results shown in Table III. This data show that the lhvehtloh: there was made a slab of the contposltlon of organopolysiloxane provided by the present invention can Eample It was 'i 2 5 135 5 be cured at significantly lower temperatures than dicumyl and then heat'age or d b ZE at i of peroxide. Fabricators as a result, will be able to achieve 1S pressfcure was ac lave y I y the outstanding cures provided by the use of dicumyl about 30 (p.s.1.) steam pressure. Fabricators skilled in emxide While utilizing facilife r ufino a mu h 1 Wer the art know that such condition are satisfactory for Alt e 1 O b presscuring elastomeric forming organopolysiloxane com- S 6am presure' emanve y a Flcators W a 9 6 positions utilizing benzoyl peroxide; these conditions are b to enloy the advantages of usmg'moldmg Faclhtlgs insuflicignt to satisfactorily presS Cm-e Such organopoly suitable forbenzoyl peroxide cures, while producing elassiloxane compositions utilizing dicumyl peroxide. 0 'f having shhstehhehy 1OWer eompressloh In addition to the composition of Example 2 contain- Whlle the foregolhg examples have been hlhlted t0 ing 0.2 part of tertiary-butylperoxyisopropylcarbonate only a f W 0f the y y Variables Within the Scope per 100 parts of organopolysiloxane composition, slabs of the present invention, it should be understood that the were prgparad f the same organopmysiloxane present invention is directed to the production of a much position using benzoyl peroxide in place of tertiary-butylbroader Class Of organopolysiloxane compositions e011- peroxysiopropylcarbonate and other slabs were prepared Vertlble the Cured, elastomefle State- These from the same composition using dicumyl peroxide as the sanopolysiloxane composltions can be made y blendmg, curing agent. Dicumyl peroxide was utilized at 0.32 part or milling together the tertiary-alkylperoxyalkylcarper 100 parts of organopolysiloxane composition, and bonates of Formula 1, with the organopolysiloxane polybenzoyl peroxide was utilized at 0.4 part per 100 parts of mers of Formula 2, as well asusmg fillers and other organopolysiloxane composition. These slabs were also ingredients shown .lII the foregolng description. press-cured and heat-aged under the same conditions. What I claim as new and desire to secure by Letters Table III shows the results obtained, where the terms Patent of the United States is: employed have the same meaning as employed above. 1. An organopolysiloxane composition compnsmg (l) 100 parts of an organopolysiloxane polymer convertible to the cured, solid, elastic state having a viscosity of at least 100,000 centipoises at 25 C. of the formula,

(R)aSiO 2 (2) 10 to 300 parts of filler, and (3) 0.001 to 10 parts of a peroxide having the formula,

(RMCOOPJOR where R is a member selected from the class consisting of monovalent hydrocarbon radicals, halogenated monovalent hydrocarbon radicals, and cyanoalkyl radicals, R is a monovalent alkyl radical having up to 8 carbon atoms, and a is equal to 1.95 to 2.01, inclusive.

2. A composition in accordance With claim 1 in which the tertiary-alkylperoxyalkylcarbonate is tertiary-butylperoxyisopropylcarbonate.

3. A composition in accordance with claim 1 in which the filler is a silica filler.

References Cited by the Examiner UNITED STATES PATENTS 2,374,789 5/1945 Strain 260-610 OTHER REFERENCES 8. Fordharn: Silicones, George Newes Ltd., London 1960. Pages 155-59. 164-69 relied upon.

F. Strain et al.: J. Am. Chemical Society, 72, March 1950; pp. 1254, 1259.

MORRIS LI'EBMAN, Primary Examiner.

I. E. CALLAGHAN, Assistant Examiner.

Claims (2)

1. AN ORGANOPOLYSILOXANE COMPOSTION COMPRISING (1) 100 PARTS OF AN ORGANOPOLYSILOXANE POLYMER CONVERTIBLE TO THE CURED, SOLID, ELASTIC STATE HAVING A VISCOSITY OF AT LEAST 100,000 CENTIPOISES AT 25*C. OF THE FORMULA, (R'')A-SI-O(4-A/2) (2) 10 TO 300 PARTS OF FILLER, AND (3) 0.001 TO 10 PARTS OF A PEROXIDE HAVING THE FORMULA, (R)3-C-O-O-COO-R WHERE R IS A MEMBER SELECTED FROM THE CLASS CONSISTING OF MONOVALENT HYDROCARBON RADICALS, HALOGENATED MONOVALENT HYDROCARBON RADICALS, AND CYANOALKYL RADICALS, R IS A MONOVALENT ALKYL RADICAL HAVING UP TO 8 CARBON ATOMS, AND A IS EQUAL TO 1.95 TO 2.01, INCLUSIVE.

3. A COMPOSITION IN ACCORDANCE WITH CLAIM 1 IN WHICH THE FILLER IS A SILICA FILLER.